US government scientists yesterday raised questions about claims of vast plumes of oil hovering beneath the surface of the Gulf of Mexico as they issued a report saying that 75% of the worst spill in American history has been cleaned up or naturally dispersed.

The White House said it had a "high degree of confidence" in the findings. But critics were swift to point out that the estimates still leave about 1m barrels of oil in the Gulf – more than the entire size of the country's second worst oil spill – while the report shows that only about one-third of the oil was actually removed by the vast cleanup effort.

According to the NOAA report, burning, skimming and pumping from the wellhead to ships removed about 25% of the estimated 5m barrels of oil that poured into the sea for three months after an explosion on the Deepwater Horizon rig that killed 11 workers. A similar amount "naturally evaporated or dissolved".

The NOAA report said: "Just less than one quarter (24%) was dispersed (either naturally or as a result of operations) as microscopic droplets into Gulf waters.

"The residual amount – just over one quarter (26%) – is either on or just below the surface as light sheen and weathered tarballs, has washed ashore or been collected from the shore, or is buried in sand and sediments. Oil in the residual and dispersed categories is in the process of being degraded."

The report was compiled with the assistance of more than 25 of the US's best government and independent scientists.

It says that 5% of the oil was burned, 3% skimmed and 8% removed by chemical dispersant. About 17% was removed after BP was finally able to attach a pipeline to the wellhead a mile below the surface of the sea and pump it on to ships.

The NOAA said dispersal of the oil was helped by local conditions and the relatively light oil spewing from the well.

"It is well-known that bacteria that break down the dispersed and weathered surface oil are abundant in the Gulf of Mexico, in large part because of the warm water, the favourable nutrient and oxygen levels and the fact that oil enters the Gulf of Mexico through natural seeps regularly," the agency said.

In contrast, another big disaster, the 1989 Exxon Valdez spill, was made worse by heavier oil and colder seas and climate.

The NOAA's assertions will raise questions about early claims by some environmentalists that vast plumes of oil were hovering below the sea surface in the Gulf. BP had claimed that they did not exist.

The report does not describe any such plumes, speaking instead of "diffuse clouds".

"All of the naturally dispersed oil and some of the oil that was chemically dispersed remained well below the surface in diffuse clouds where it began to dissipate further and biodegrade," it said.

"Previous analyses have shown evidence of diffuse clouds of dispersed oil between 3,300 and 4,300ft [1,000-1,300 metres] in very low concentrations (parts per million or less), moving in the direction of known ocean currents and decreasing with distance from the wellhead."

Yesterday, BP said that it has not made its own assessments of the environmental impact of the spill and that others would have to draw their own conclusions about whether claims of vast oil plumes floating under the sea's surface were accurate in light of the NOAA report.

But even as the threat from the spill is receding, there are continued questions over the spraying of 1.8m gallons of chemical dispersant, which breaks the oil into tiny particles that hover below the water's surface, where natural bacteria eat them.

The Deepwater Horizon spill was nearly 20 times larger than the Exxon Valdez disaster but so far it has affected a much smaller area of coastline. Officials say that this is largely due to the spraying of dispersant.

Some scientists say that the effects of the dispersant are less harmful, or no worse, than the oil itself.

The US Environmental Protection Agency released test results earlier this week showing that the impact of dispersant on shrimp and fish were not noticeably more toxic than the effect of oil.

But some environmentalists say that the tests do not measure the longterm effects of chemicals on sealife.

The US Senate's environment committee yesterday began a hearing on BP's use of dispersant. In May, the EPA ordered the company to greatly reduce its use of the chemicals and to cease spraying them on the water's surface except in "rare cases".

But the agency relaxed its ruling as more oil washed up on the coast.

Scientists at the NOAA are preparing a report on the long-term effect of dispersant which is likely to figure in negotiations between BP and Washington over compensation payments for cleaning up the environment.